Microwave energy heats many substances much more rapidly and uniformly than older heating techniques. Consequently, a variety of microwave heating chambers have been developed for processing industrial and agricultural products that require heat treatment. Such heating chambers are often equipped with a moving belt or other type of conveyor for carrying substances through the microwave region on a continuous process basis. Cooking of food products, drying materials and curing of plastics are among the many usages of conveyorized microwave heating chambers.
Safety considerations and the need to avoid interference with nearby electronic equipment dictate that the microwave energy be substantially wholly contained within the heating chamber. In order to contain the microwave energy it has been the prior practice to enclose the heating region with electrically conductive metal walls except at the conveyor access openings which in some installations are metal tunnels lined with microwave absorbent dielectric material.
Metal has appeared to be the obvious material for defining a microwave energy containment region as such energy is reflected by electrically conductive surfaces. Microwave energy penetrates non-conductive materials at least for substantial distances. Some non-conductive materials are electrically lossy dielectrics in which microwave energy is gradually absorbed. A thickness of at least about 20 centimeters is required for full absorption of industrial microwave frequencies by such lossy dielectric materials.
Microwave energy is costly to produce and absorption of such energy by anything other than the substance on the conveyor is a costly inefficiency in the heating operation. Thus absorbent dielectrics have not heretofore been believed to be desirable materials for defining the microwave energy containment region. The conductive metal walls of prior heating chambers enhance efficiency by repeatedly reflecting energy, that may have passed through the product unabsorbed or that may have bypassed the product, until such time as it is absorbed in the product.
While being very effective from the functional point of view, conveyorized microwave heating chambers of the above discussed kind also tend to be undesirably costly, particularly when the costs of shipping the apparatus from the factory to a distant point of use are taken into account. The metal walled heating chambers may require expensive materials and must be carefully manufactured to assure that there are no potential leakage sites for microwave energy. Finished components are bulky and require careful handling during transport to the installation site. Such cost considerations may have inhibited usage of microwave heating in industrial operations. The problem is particularly acute where very large scale heating operations are required as correspondingly large heating chambers and conveyor systems are needed.
Unlike the chamber itself, the conveyor in most prior microwave heating installations of this kind is formed of non-conductor. Such conveyors are typically a belt or a vibrating trough formed of plastic material although conveyors formed of conductor such as metal are in general relatively stronger, more durable, more easily cleaned and are capable of withstanding higher temperatures. Several factors have heretofore tended to limit usage of conductive conveyors in microwave heating systems.
As previously pointed out, conveyor access to many prior microwave heating chambers is provided for by metal tunnels lined with lossy dielectric material through which the conveyor enters and leaves the microwave containment region. Microwave energy entering such tunnels from the containment region does not propagate along the axis of such tunnels in a straightforward manner. Such energy tends to spread out as it progresses along the tunnel. Consequently, it is repeatedly intercepted by the metal walls of the tunnel and reflected back towards the opposite walls. This causes such energy to repeatedly pass through the lossy dielectric lining of the tunnel which in turn results in a progressive attenuation of such energy as it travels along the tunnel. This is a highly advantageous access structure as it provides large, continually open passages into and out of the heating chamber while suppressing escape of microwave energy.
If a conductive conveyor belt, for example, extends through such an access tunnel in parallel relationship to the metal walls of the tunnel, the effect is to divide the tunnel into two parallel waveguides or microwave conductors. Attenuation of the microwave energy will still occur owing to the presence of some of the dielectric lining in each such waveguide but the attenuating effect is greatly reduced. At best, the tunnels must be elongated or other undesirable structural complications introduced if a conductive conveyor is to be run through such a tunnel.
Another consideration which has inhibited use of conductive conveyors arise from the basic physical fact that electrical field intensity is always essentially zero at a conductive surface that bounds a microwave region. A difference in electrical potential or voltage cannot exist between two points on the surface of a conductor. It can readily be observed that the metal walls of a microwave oven are not directly heated by the microwave energy aside from some resistance heating by induced wall currents. This has led to a general assumption that direct heating by microwave energy will not occur in that part of a dielectric substance that is in actual contact with the surface of a conductive conveyor and that only reduced heating will occur for a small distance outwardly from such a conveyor surface.
Other seemingly adverse characteristics of conductive conveyors include the fact that most electrical conductors are also excellent thermal conductors. An electrically conductive belt or the like is not itself directly heated by microwave energy and thus is capable of conducting heat out of substances which rest on the conveyor. Electrical arcing can also present problems as damaging, energy dissipating sparks can occur between a metal belt and adjacent conductive chamber walls in a microwave field unless sizable spacing is provided by reducing the width of the belt relative to that of the chamber itself.
It would be advantageous if the foregoing problems were resolved to enable more widespread use of conductive conveyors which, as previously discussed, have a number of very desirable characteristics. If the foregoing problems are overcome, conductive conveyors such as metal belts would be more useful in installations of any size but this is particularly the case in very large heating installations where product loads and processing rates may be beyond the capabilities of plastic belts or the like because of the limited structural strength of such materials.
Resolution of the foregoing problems would also enable use of a single lengthy high strength metal conveyor for carrying products through other work stations as well as a microwave heating chamber and thereby reducing handling and transfering of the product between a series of separate conveyors. As one example, industrial precooking of bacon may involve movement of the product from a slicer to a steam precooker and then through a microwave heating chamber to a packaging station. A series of plastic belts has been used for this purpose because of the strength limitations of such materials. Sliced bacon is fragile and transferring of the product between belts has been a significant source of product loss.
The present invention is directed to overcoming one or more of the problems set forth above.